One consideration for the performance of a power amplifier is its ability to exhibit the gain and phase response which is requested and/or expected of it. In most applications, a temporal variance of the gain and phase response of a PA from what is required and expected is undesirable. An input signal to the PA experiences amplification in accordance with the expected gain and the phase of the signal may be shifted. A temporal variance in the gain or phase shift experienced by the input signal results in a temporal variance in both the magnitude and phase of the output signal.
In a PA used in an RF transmitter or transceiver for RF communication, one measure of the error in the gain and phase response exhibited by a PA is what is known as the error vector magnitude (EVM) of the RF signals transmitted. The EVM is characterized by the magnitude of error in the transmitted signal symbol's constellation points versus the constellation point locations of the input signal symbol. The EVM performance of a PA can be measured in terms of the contribution to the EVM of the transmitter created by the amplification applied by the amplifier. All PA's contribute to the EVM to some degree as no PA is an ideal amplifier. Keeping the EVM caused by a PA as small as possible is an important goal in the design and manufacture of PAs.
The EVM of a PA can increase under pulsed conditions. Dynamic EVM is a measure of this increased EVM under these conditions. A significant contributor to dynamic EVM is variance in at least one of the gain or phase response of the PA when the PA is experiencing transience, particularly during transitions from idle to steady-state PA operating conditions.
Under steady-state operating conditions, the PA generally behaves in an expected, well behaved, and settled manner. Most PAs are designed and tuned such that, under steady-state conditions, if a specific bias current (or voltage) is applied to its amplifying circuitry from associated biasing circuitry, it amplifies a signal with a predetermined and settled gain and phase response. This invariance of gain and phase response holds in general if the PA is operating under steady-state operating conditions, but does not necessarily apply under changing operating conditions such as at initial biasing or PA turn-on.
Not surprisingly, changing operating conditions can lead to changing electromagnetic, electrical, and other physical characteristics of the PA, its constituent components, or its associated circuitry. Changes in these physical characteristics can lead to an exhibited gain or phase response of the PA which does not correspond to that which normally accompanies the specific bias being applied to the PA. If a physical characteristic of the PA fluctuates, and if the gain or phase response of the PA is affected by that physical characteristic, so will the exhibited signal amplification response of the PA. As such, changing the operating conditions or operating point of the PA can lead to changes in the gain or phase response of the PA. For example, transitioning from an idle state to a full-on state can create a host of various, and possibly interdependent or causally related physical changes which lead to temporal variance in the gain response of the PA and increased dynamic EVM, during the transition period. One important operating condition which affects the gain or phase response of the PA and which changes rapidly during transitions from an idle state to a full on-state is temperature.
In order to ensure low dynamic EVM and provide for an output signal that is not distorted by gain or phase variance, a PA is generally not used unless it is thermally settled. A common approach to avoid the problem of dynamic EVM is simply to wait until a PA is thermally settled before using it to amplify the signal. This approach may not be acceptable in the context of various radio transmission standards such as IEEE 802.11. A second common approach is to reduce the time needed to achieve a steady state in respect of the amplification characteristics of the PA (“settling time”) by applying an external resistor and speed-up capacitor to provide more forward current earlier to the PA. Although the speed-up capacitor can improve the settling time of the PA, as a passive mechanism it cannot provide the additional forward current until the RF input signal itself arrives. Consequently, the beginning of the RF signal data will suffer from some amount of dynamic EVM and the additional current may not be sufficient to bring the PA into a thermally settled state at a desired rate.
The external passive networks according to the known solutions possess various packaging and performance compromises. Use of the speed-up capacitor requires additional package pins to connect the capacitor across the bias reference current internal port. Any solution utilizing fixed external passive networks requires extensive fine tuning and optimization in the prototype phase. Waiting for a PA and its associated circuitry to warm-up and become thermally settled before using it, although avoiding dynamic EVM, introduces undesirable delay.